This invention relates to a pressure-sensitive input apparatus that operates by detecting variations in electrical resistance, and particularly to an improvement for detecting input at multiple locations.
A prior-art example of a pressure-sensitive input apparatus that operates by detecting variations in electrical resistance is described in Technical Paper of the Institute of Electronics and Communications Engineers of Japan IE81-7, May 1981, pp. 1-6 published by the Institute of Electronics and Communications Engineers of Japan. This apparatus is capable of detecting the positional coordinates of an input at a single point only on a pressure-sensitive input surface.
FIG. 1 shows the typical principle of operation of a prior art pressure-sensitive input apparatus that operates by detecting variations in electrical resistance. This input apparatus comprises an input panel N which includes a group of vertical electrodes 1 and a group of horizontal electrodes 2, which extend orthogonally with respect to the vertical electrodes 1. Disposed between the vertical electrodes 1 and the horizontal electrodes 2 is a pressure-sensitive sheet 5. The electrical resistance of the pressure-sensitive sheet 5 is reduced at location where a pressure is applied in the thickness direction. As an alternative, the vertical electrodes 1 and the horizontal electrodes 2 are separated by an air gap, in which case the vertical electrodes or the horizontal electrodes are resiliently deformed until they are in contact with each other. In both groups, the electrodes 1 and 2 are disposed parallel to one another, and the pitch of the electrodes corresponds to the resolution of the input apparatus. Connected to these groups of electrodes 1 and 2 and disposed orthogonally to each other are an X-coordinate detection resistive member 3 and a Y-coordinate detection resistive member 4, both of which have a uniform lengthwise resistivity (resistance per unit length). The horizontal electrodes 2 are covered with a protective, insulating sheet, not shown. The details of the structure of the input panel N is known in the art. Examples of the input panel are shown in Japanese Patent Application Publication No. 34543/1982, which is hereby incorporated by reference and further explanation thereof is omitted.
A constant-current source 6 is further connected as shown in the drawing, and current detection circuits 7 and 8 are provided. A positional coordinate calculation circuit 10 is provided to receive the outputs from the current detection circuits 7 and 8 to calculate the positional coordinate of the point P at which the stylus S is applied.
The apparatus operates as follows. Suppose there is an input at a point P located at a position x on the X axis and y on the Y axis, as measured from the origin O of the effective input area. At the point P at which pressure is applied, the electrical resistance in the thickness direction of the pressure-sensitive sheet (or air gap) 5 is reduced, so the electrodes in the group of vertical electrodes 1 and the group of horizontal electrodes 2 at this point are connected through a contact resistance r.sub.c. The equivalent circuit at this moment is shown in FIG. 2, in which R.sub.x is the total resistance of the X-coordinate detection resistive member 3, R.sub.y is the total resistance of the Y-coordinate detection resistive member 4, I.sub.o is the current value of the constant-current source 6, and P.sub.x and P.sub.y are the points on the X-coordinate and Y-coordinate detection resistive members 3 and 4, respectively, at which the vertical and horizontal electrodes 1 and 2 are connected.
The following relationship is easily derived from the equivalent circuit in FIG. 2. EQU x=L.sub.x .multidot.i.sub.2 /I.sub.o ( 1) EQU y=L.sub.y .multidot.i.sub.4 /I.sub.o ( 2)
where L.sub.x and L.sub.y denote dimensions of the effective input area in the x- and y-directions, respectively. Since L.sub.x, L.sub.y, and I.sub.o are constants, x is directly proportional to i.sub.2 and y is directly proportional to i.sub.4. The positional coordinate (x, y) of the point P at which pressure was applied can therefore be found by measurement of the current values i.sub.2 and i.sub.4 by the current detection circuits 7 and 8. An important point to note is that the position can be detected independently of the contact resistance r.sub.c between the vertical and horizontal electrodes 1 and 2.
A problem in the apparatus described above is that when there is input at multiple points, which may occur when the input surface is touched by simultaneously the input stylus S and another object such as a finger at a pressure exceeding the threshold value, it is not possible to detect the correct position. Moreover, the prior-art apparatus is not able to detect that a multiple input has occurred. Accordingly, a value quite different from the coordinate of the position intended by the operator may be erroneously obtained and supplied to a data processing device connected to the pressure-sensitive input device.